The air you breathe may hurt your ears.

The air you breathe may hurt your ears

Patrons of smoky honky-tonks may subject their ears to more than just music, and mechanics who inhale exhaust while working around noisy engines may take home more than grease under their fingernails. Ongoing research suggests prolonged exposre to high carbon monoxide levels combined with loud noise can cause permanent hearing damage.

Smokers carry 300 to 400 parts per million (ppm) of carbon monoxide in their lungs during smoking, says Laurence D. Fechter of Johns Hopkins University in Baltimore. For 3-1/2 hours, he and co-workers subjected rats to carbon monoxide concentrations of 500 ppm, a dose he says "might correspond to human smoking." During the last 2 hours, the researchers also exposed the rats to a constant noise level of 105 decibels (dB), roughly that experienced by front-row fans at indoor rock concerts.

The rats suffered permanent hearing loss averaging 20 dB from a single exposure. "Loss of the same magnitude in humans would impair their ability to hear conversation," Fechter says. The group plans epidemiologic studies to determine how human hearing responds to the combination of noise and smoke.

Presented this week in Honolulu at a meeting of the Acoustical Society of America and its Japanese counterpart, the results expand on the team's earlier finding that loud noise and 1,200 ppm of carbon monoxide together pose a far greater hearing-loss risk than either alone. Scheduled for publication this winter in FUNDAMENTAL AND APPLIED TOXICOLOGY, the new findings also suggest carbon monoxide and other chemicals cause hearing damage by restricting the flow of oxygen to nerve cells in the cochlea of the inner ear. These "hair cells" demand more oxygen as sound level increases, Fechter says.

Earlier studies have shown a few chemicals can induce temporary hearing loss without any coincident noise (SN: 5/22/82, p.347). Now the Hopkins group has demonstrated that butyl nitrite, a street drug commonly called poppers, by itself can cause longer-term hearing damage. The team's most recent measurements show rats averaged 20 dB of hearing loss one week after getting 70 microliters of the drug per 100 grams of body weight. Fechter says this dose exceeds what people probably would inhale, but butyl nitrite users tend to breathe the drug in loud environments -- such as discotheques and concert halls -- that compound hearing-loss risk.

The researchers also have shown that high doses of trimethyltin chloride, found in some pesticides and recently banned marine paints, by themselves can damage rats' hair cells irreversibly. The group plans experiments to determine whether butyl nitrite can permanently impair rats' hearing and to clarify the oxygen-restriction process.